1. Field of the Invention
The present invention is in the field of calcination of fine grained material, particularly of the type used for cement clinker, utilizing two essentially parallel preheater lines, with means being provided to more efficiently divide the distribution of material between the lines and providing more efficient control of the agglomeration and calcination behavior of the meal being processed.
2. Description of the Prior Art
This invention is an improvement of the subject matter described in U.S. Ser. No. 635,894, filed July 30, 1984, and based upon German patent application No. P 33 27 576.9, filed July 30, 1983.
As explained in the prior application, the provision of essentially parallel product streams utilizing separate preheating lines is known to the prior art. For example, there is shown an apparatus for performing this method in the article "Experience With Pre-Calcining Taking Into Account Substitute Fuels" appearing in the Journal "Zement-Kalk-Gips", No. 5, 1979, page 218, picture 12. In the apparatus illustrated in this article, cooler exhaust air and furnace exhaust air are utilized in two separate preheating lines. A calcinator line containing four cyclone stages is followed by a calcinator to which combustion air from the cooler is supplied. A parallel furnace line includes a conventional four-stage cyclone preheater. The meal is distributed between the preheater lines approximately proportionately to the gas quantities. For example, one calcinator line receives approximately 62% and the other about 38% of the meal. In the calcinator, the meal streams from the fourth stage of the furnace line and the third stage of the calcinator line flow together. Beneath the meal inlet a sufficient amount of heating oil is blasted in such that approximately 90% deacidification is obtained. Exhaust gas and deacidified meal are drawn off from the calcinator, subsequently directed into the fourth cyclone stage of the calcinator line, and then separated from one another therein. There is an advantage in this type of construction in that the separate gas guidance permits the separate regulation of two preheater lines and permits variation of performance within a greater range.
In the improved form of this type of apparatus described in the aforementioned copending application based on German patent application No. P 33 27 576.9, the material is thermally treated in a preheating zone into separate, essentially parallel streams and then is calcined in a clinker-forming zone. One of the preheater lines is connected to the exhaust gas conduit of the clinker-forming zone and the other preheater line is contacted with a stream of reaction gas from the calcination zone which in turn is charged with hot exhaust air from the cooler zone together with fuel. The stream of material preheated in the exhaust air of the clinker-forming zone is divided in the area of the preheating zone into two partial streams one of which is conveyed back into the stream of exhaust gas of the clinker-forming zone and the other partial stream is introduced into the calcination zone.
In the methods and apparatus of the type mentioned previously, cooler exhaust air and kiln exhaust gas are converted into two separate preheater lines one of which contains a calcinator connected in series with the cyclone heat exchanger stages in the direction of flow of the material, to which there is conveyed combustion air from the cooler, together with preheated powder and fuel. In this type of system there is usually a four-stage cyclone preheater attached to the exhaust gas conduit from the clinker kiln. A crude powder is allocated somewhat proportionately to the quantities of gas passing through the various apparatus whereby, for example, the calcinator line may contain approximately 60% and the kiln or furnace line contain about 40% of the quantity of powder being treated. The methods and apparatus of this type according to the present state of the art are in most cases so disposed and operated that in the calcinator, streams of powder flow together out of both preheating lines. In the area of the powder inlet, fuel is added to the calcinator in such quantity that almost 80 to 90% deacidification is achieved. Exhaust gas and deacidified powder are separated in a separation stage in series with the calcinator, and the calcined material is conveyed into the inlet of the rotary kiln for clinker production. An advantage of this type of construction resides in the fact that both preheating lines may be regulated separately and the yield may be altered over a greater range.
There was a difficulty found, however, in attempting to synchronize the rate of precalcination of both lines in an optimal manner. The difficulty occurred in that in the preheating line connected to the exhaust gas of the clinker-forming reactor, a dust circuit developed wherein portions of dust-type materials were introduced which were completely deacidified. Such portions of material passing from the preheating line into the calcination zone possessed a higher degree of deacidification than portions of the material exiting from the calcinator. For the prevention of over-calcination and the disadvantage of caking in the calcination zone, the addition of fuel in the calcination zone could not be undertaken at a level which would be necessary for deacidification as far as possible of all material parts which would be thermally pretreated in the preheating line acted on by the air from the cooler.
The difficulty would be particularly pronounced with installations which were originally laid out with only one preheating line and by means of subsequent addition, a second preheating line was added with a calcination device to substantially increase the yield.
In the prior application these mentioned difficulties were overcome by adapting the rate of precalcination of both lines optimally to one another. Consequently, methods and installations of the type mentioned were far improved and developed that prevention of over-calcination and/or formations of deposits were essentially prevented, and a substantially uniform degree of deacidification was achieved in the materials which were pretreated and precalcined in both the parallel preheating lines before their introduction into the clinker-forming reactor.
Another advantage was that the increase in the degree of deacidification in the preheater line fed by the stream of exhaust gas from the clinker reactor was achieved without the necessity of adding fuel, whereby the danger of a CO emission in the exhaust gas was prevented.